2 Answers

+3 votes
answered Aug 19, 2014 by cchang219 (2,310 points) | selected Aug 30, 2014 by odadmin
 
Best answer

Void spaces used for temporary ballast should be coated but should not be equipped with a cathodic protection system due to the short duration of the temporary ballasting procedure.

For other considerations, the hull should be coated in the splash zone, but should not be coated below the splash zone.

External surfaces of the hull below the splash zone should receive corrosion protection from a sacrificial anode type cathodic protection system comprised of aluminum-indium-zinc alloy anodes. The system should be designed to provide protective current to all submerged steel surfaces of the hull and its appurtenances for a design life of 25 years. The cathodic protection system should be designed in accordance with the requirements of DNV RP-B401 and applicable project documents.

Hull anodes should be pipe-core stand-off and flush mount type.

In addition to the hull, submerged portions of the following structures/components should receive cathodic protection from external hull mounted galvanic anodes:

  • Top tension production and water injection risers
  • Future top tension production and water injection risers
  • Drilling riser
  • Flexible fluid transport lines
  • Control umbilical
  • Caissons (external)
  • Tendon porches
  • FTL porches
  • Boat landing
  • Miscellaneous hull appurtenances

Caissons should be externally coated in the splash zone with the same coating system used for hull splash zone protection. The submerged portions of the caissons should be equipped with a 2-3 coat epoxy coating system suitable for immersion under cathodic protection.
Ballast tanks should be equipped with a protective coating system, in addition to a sacrificial anode cathodic protection system. Anodes should be stand-off type, except as required to address congested areas or cathodic shielding.

+2 votes
answered Aug 19, 2014 by sicalamanto (1,560 points)

The effect of coatings on the design of the cathodic protection system must also be considered.

Coatings reduce cathodic protection requirements over the system life cycle and improve current distribution, particularly in congested or shielded areas. Initial, mean and final coating breakdown factors for structures coated with a 2-3 coat epoxy system should be formulated based on guidelines provided in DNV RP-B401, in conjunction with an analysis of installation, operation, and maintenance/monitoring considerations. Coating breakdown factors for TLPE (tendons) and 5LPP (TTRs) should be formulated based on guidance from ISO-15589-2. Compatibility with cathodic protection should be considered in the coating selection process for submerged surfaces, such as the internal surfaces of the variable ballast tanks.

Production system components generally experience operating temperatures up to 121°C (250°F), and corrosion rates typically increase with temperature, as do cathodic protection current requirements. The efficiency (capacity) of aluminum-indium-zinc alloy anodes decreases dramatically at temperatures in excess of 30º C (86°F).

For piping and other system components with surface temperatures in excess of 25º C (77°F), the initial, mean and final design current density values should be adjusted in accordance with applicable standards including DNV RP-F103 and ISO-15589-2.

Anode attachment locations should be chosen so as to assure that the temperature of the anode material does not exceed 30º C (86°F). Where feasible, anodes should be attached to structural members and not directly to elevated temperature components such as piping.

Establishing and maintaining electrical continuity between the hull and components requiring cathodic protection from hull based anodes components over the design life is critical to the success of the cathodic protection design strategy.

All non-welded connections within component assemblies should be tested for electrical continuity. 

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